Microlithography (also referred to as photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. The process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) or vacuum ultraviolet (VUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes a light source, an illumination system that illuminates the mask with projection light produced by the light source, a mask stage for aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of a rectangular or curved slit, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go. Such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection beam along a scan direction while synchronously moving the substrate parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection objective, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning device. Commonly used masks contain opaque or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror or LCD array.
As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illumination field on the mask with projection light having a well defined irradiance and angular irradiance distribution. The term angular irradiance distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions of the rays that constitute the light bundle.
The angular irradiance distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. Often the optimum angular irradiance distribution depends on the size, orientation and pitch of the features contained in the pattern. The most commonly used angular irradiance distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil plane of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil plane. Thus there is only a small range of angles present in the angular irradiance distribution of the projection light, and all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular irradiance distribution of the projection light in the mask plane so as to achieve the desired illumination setting. In the simplest case a stop (diaphragm) including one or more apertures is positioned in a pupil plane of the illumination system. Since locations in a pupil plane translate into angles in a Fourier related field plane such as the mask plane, the size, shape and location of the aperture(s) in the pupil plane determines the angular irradiance distributions in the mask plane. However, any change of the illumination setting involves a replacement of the stop. This makes it difficult to finely adjust the illumination setting, because this would involve a very large number of stops that have aperture(s) with slightly different sizes, shapes or locations. Furthermore, the use of stops inevitably results in light losses and thus in a reduced throughput of the apparatus.
Many common illumination systems therefore include adjustable elements that make it possible, at least to a certain extent, to continuously vary the illumination of the pupil plane. Often these systems use an exchangeable diffractive optical element to produce a desired spatial irradiance distribution in the pupil plane. If zoom optics and a pair of axicon elements are provided between the diffractive optical element and the pupil plane, it is possible to adjust this spatial irradiance distribution to some extent.
Recently it has been proposed to use mirror arrays that illuminate the pupil plane. In an illumination system known from EP 1 262 836 A1 the mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Each of the mirrors can be tilted in two different planes perpendicular to each other. Thus radiation incident on such a mirror device can be reflected into almost any desired direction of a hemisphere. Each mirror produces a spot in the pupil plane that is freely movable by tilting the mirror.
Similar illumination systems are known from US 2006/0087634 A1, U.S. Pat. No. 7,061,582 B2, WO 2005/026843 A2 and WO 2010/006687 A1.
As mentioned further above, it is usually desired to illuminate, at least after scan integration, all points on the mask with the same irradiance and angular irradiance distribution. If points on the mask are illuminated with different irradiances, this usually results in undesired variations of the critical dimension (CD) on wafer level. For example, in the presence of irradiance variations the image of a uniform line on the mask on the light sensitive may also have irradiance variations along its length. Because of the fixed exposure threshold of the resist, such irradiance variations directly translate into widths variations of a structure that shall be defined by the image of the line.
If the angular irradiance distribution unintentionally varies over the illumination field on the mask, this also has a negative impact on the quality of the image that is produced on the light sensitive surface. For example, if the angular irradiance distribution is not perfectly balanced, i.e more light impinges from one side on a mask point than from the opposite side, the conjugate image point on the light sensitive surface will be laterally shifted if the light sensitive surface is not perfectly arranged in the focal plane of the projection objective.
In the following some approaches are described how the angular irradiance distribution can be modified in a field dependent manner.
WO 2012/100791 A1 discloses an illumination system in which a mirror array is used to produce a desired irradiance distribution in the pupil plane of the illumination system. In close proximity to the pupil plane a fly's eye optical integrator is arranged that has a plurality of light entrance facets. Images of the light entrance facets are superimposed on the mask. The light spots produced by the mirror array have an area that is at least five times smaller than the total area of the light entrance facets. This makes it possible to produce variable light patterns on the light entrance facets, and thus different angular irradiance distributions at different portions of the illumination field. For example, at one portion of the illumination field an X dipole and at another portion of the illumination field a Y dipole illumination setting may be produced.
WO 2012/028158 A1 discloses an illumination system in which the irradiance distribution on the light entrance facets of the fly's eye optical integrator is modified with the help of a plurality of modulator units that are arranged in front of the optical integrator. Each modulator unit is associated with one of the light entrance facets and variably redistributes, without blocking any light, the spatial and/or angular irradiance distributions on the associated light entrance facet. In this manner it is possible, for example, to illuminate, with different illumination settings, two or more different portions on a single die that are associated with different semiconductor devices.
WO 2015/074746 A1 discloses an approach in which the irradiance distribution on the light entrance facets of a fly's eye optical integrator is modified by imaging a digital mirror device (DMD) on the light entrance facets. This approach is advantageous because it is not necessary to produce very small light spots with an analog micromirror array, as this is the case in the illumination system known from WO 2012/100791 A1 mentioned above. The field dependency of the angular irradiance distribution is adjusted so that the angular irradiance distribution over the illumination field becomes perfectly uniform (i.e. field independent). It is also mentioned that it may sometimes be desirable to deliberately introduce a field dependency of the angular irradiance distribution. This may be expedient, for example, if the projection objective or the mask has field depending properties. As far as the mask is concerned, such field depending properties are usually a result of features that have different orientations or dimensions. Adverse effects resulting from such field dependencies can be successfully reduced by selectively introducing a field dependency of the angular irradiance distribution.
However, it turned out that occasionally the angular irradiance distribution produced with the illumination system known from the aforementioned WO 2015/074746 A1 slightly deviates from the expected target distribution although the digital mirror device has been correctly controlled.